The present invention relates to novel compounds of the phenylcyclohexyl dicarboxylate type and to the preparation process for these.
More specifically, it relates to producing novel liquid crystals or non-mesomorphic compounds able to improve the elastic behavior of nematic liquid crystals usable in devices making use of the electrically controlled birefringence effect.
This effect corresponds to a deformation under an electric field of a nematic phase with negative dielectric anisotropy .DELTA..epsilon., the latter representing the difference between the dielectric constant .epsilon..sub.a parallel to the major molecular axis of the crystal and the dielectric constant .epsilon..sub.b perpendicular to said axis.
In liquid crystal devices, it is necessary to use nematic materials permitting a high degree of multiplexing, i.e. a large number of electrically addressable screen lines, in order to display a large amount of information. This multiplexing level k can be expressed as a function of the voltage applied to the terminals of the device (V) and the crystal deformation threshold voltage (V.sub.S) by the formula: ##EQU1##
For a given multiplexing level, it is necessary to have a molecular tilt angle (.PHI..sub.M) in the center of the liquid crystal cell which is as large as possible, in order to obtain the optimum contrast between the two black and white states of the crystal under the action of the electric field. For small tilt angles .PHI..sub.M is directly linked with the crystal deformation threshold voltage V.sub.S and the ratio of the elastic constants of bending K.sub.33 and fanning K.sub.11 of the liquid crystal. Thus, .PHI..sub.M is given by the formula: ##EQU2##
Under these conditions, the electrically induced birefringence is then of form: ##EQU3## in which n.sub.e and n.sub.o are extraordinary and ordinary indices of the crystal, e the thickness of the liquid crystal and Z its position in the cell.
The relative intensity transmitted by the liquid crystal device between two cross-polarizers is given by the relation: ##EQU4## in which .DELTA.n is equal to n.sub.e -n.sub.o and .lambda. is the wavelength of the light beam illuminating the device. Thus, the most important parameters of the material are K.sub.33 /K.sub.11, .DELTA.n, .DELTA..epsilon., as well as the mesomorphism range .DELTA.T of the crystal. Among these parameters, an important part is played by the ratio K.sub.33 /K.sub.11, because it determines the steepness of the slope of the electro-optical transfer curve and consequently the multiplexibility of the material used. This ratio must be as large as possible.